The present invention relates to microstructures of iron, nickel and chromium alloys which are stable, in particular under conditions of high temperatures (900-1050° C.) and/or of high pressures (10-40 bar), to the alloys comprising these microstructures, to the process for the manufacture of these alloys and to the reforming tubes comprising these alloys.
Alloys of this type can be used in the manufacture of reforming tubes for the production of synthesis gas (a mixture of H2 and CO), but also in the manufacture of furnaces, for example heat treatment furnaces. Reforming tubes are filled with catalyst consisting of nickel supported on alumina. The decomposition reaction of methane is endothermic and requires an external heat source, which is generally installed inside a combustion chamber equipped with burners. These operating conditions impose two main requirements on the reforming tubes, namely the tubes have to be resistant to high-temperature oxidation and, most importantly, to deformation by creep. Currently, plants use standard tubes or the microstructure is not controlled or stabilized despite the severe temperature and pressure conditions.
Under these severe conditions, the alloy can rapidly age, which will result in premature fracturing and thus in loss of production of the synthesis gas often combined with fines paid by the client for the uninterrupted provision of hydrogen and carbon monoxide.
In other words, the alloys of the reforming tubes exhibit a limited creep strength if they are exposed to temperatures of greater than 900° C.
The microstructure of the alloy is very complex and its constituents appear at different scales, as demonstrated in FIG. 1. On the macroscopic level, the grains of this type of alloy are sometimes of columnar and equiaxed type or of columnar type only but of millimetric size. On the microscopic level, a network of primary carbides is found at the limits of the dendritic cells. Due to the instability of the initial microstructure in service, a fine secondary precipitation takes place in the eutectic cell which is an austenitic matrix. Taking into account the working conditions, two creep mechanisms may be involved: diffusion creep and dislocation creep. The microstructural optimization consists in controlling the precipitation process during service since fine secondary precipitates act as a barrier to the movement of dislocations and in this way promoting a slowing down in phenomenon of deformation by creep.
The typical microstructure of these alloys in the rough state is an austenitic matrix comprising primary intergranular precipitates having a eutectic structure, such as chromium carbides of M7C3 (M=Fe, Ni, Cr) or M23C6 (M=Fe, Ni, Cr) type and niobium and titanium carbides of MCN (M=Nb, Ti) type.
Starting from that, one problem which is posed is that of providing an alloy exhibiting a better microstructure making it possible to better withstand high temperatures and pressures.